Comparing Placidus, Whole Sign and the Main House Systems

A planet appears in the second house. You change the house system and find it in the first. If its sign and degree remain the same, start by examining the house boundaries.

A house system defines how the twelve houses are constructed. Different methods can place intermediate cusps at different degrees for the same date, time, location and zodiac. Reading only the planet’s new house number can hide the reason for the change.

This guide covers the nine main choices in the Hermes house menu: Placidus, Whole Sign, Equal Houses from the Ascendant, Regiomontanus, Porphyry, Alcabitius, Topocentric (Polich-Page), Koch and Campanus. The first useful distinction is that they divide different geometrical quantities.

If you are also changing the zodiac, start with our tropical and sidereal zodiac guide. Keep the zodiac fixed for the house comparison here.

Separate the house cusp, Ascendant and MC

A cusp marks the beginning of a house. Treating the first cusp as identical to the Ascendant in every method will make Whole Sign particularly confusing.

The Ascendant (ASC) is the point of the ecliptic rising on the eastern horizon at the specified place and time. The MC (Medium Coeli) is the intersection of the ecliptic with the upper branch of the local meridian. These points are calculated; the house method determines how house boundaries relate to them.

At the ordinary latitudes used in our exercise, quadrant methods such as Placidus place the first cusp at the ASC and the tenth at the MC. Whole Sign and Equal Houses from the ASC do not share that entire arrangement. Whole Sign starts the first house at the beginning of the rising sign; Equal starts at the exact Ascendant degree. In both methods, the MC need not coincide with the tenth cusp.

The technical definitions are described in the Swiss Ephemeris documentation, §6.2–6.3.

What do the nine methods divide?

Two methods can produce evenly spaced divisions on different celestial circles. One may divide ecliptic longitude, another the celestial equator, and another a point’s day or night arc. Equal divisions made in one plane need not produce equal intervals on the ecliptic.

The celestial equator is Earth’s equator projected onto the celestial sphere. The prime vertical is the great circle passing through east, the zenith directly overhead, and west. Keep those two circles separate when comparing Regiomontanus with Campanus.

The table maps the nine main Hermes menu choices to the method definitions in Swiss Ephemeris §6.2. You can use the letters in the table as shorthand for the methods in your notes.

“Topocentric” deserves particular care. The Topocentric house system is a method of constructing houses. Calculating a planet from the observer’s location rather than Earth’s centre is a separate calculation choice. Selecting T in the house menu does not establish that every planetary position has changed its observation centre.

Work through Whole Sign versus Equal Houses

Assume an Ascendant of 18° Leo. We are deriving boundaries from the two definitions, rather than calculating a birth chart.

Whole Sign starts the first house at 0° Leo. Equal Houses from the ASC starts it at 18° Leo, then advances by 30° for each subsequent cusp.

Now add an illustrative planet with an ecliptic longitude of 5° Virgo:

• In Whole Sign, Virgo is the second house, so the planet is in the second house.
• In Equal Houses, the first house extends from 18° Leo to 18° Virgo, so the planet remains in the first house.

The planet’s longitude has stayed fixed. Moving the house boundary changes its house number. Both methods use 30° houses, but they begin those houses at different places.

Why can Placidus and Porphyry disagree?

Sharing the Ascendant as the first cusp and the MC as the tenth does not make the intermediate cusps identical. The methods construct the divisions between those points differently.

Porphyry is easy to illustrate geometrically. Suppose the relevant ecliptic quadrant running from the MC towards the ASC spans 96°. Dividing by three gives 32° per house in that quadrant, with the eleventh and twelfth cusps 32° and 64° along it. An adjoining quadrant of 84° would contain three divisions of 28°. All twelve houses need not have the same width.

Placidus does not obtain those intermediate cusps by simply trisecting the ecliptic quadrant. The four main boundaries can therefore agree while intermediate cusps—and the houses of planets near them—differ.

The 96° and 84° arcs are teaching values. They are not Hermes results for the synthetic date and location below.

Set up a comparison in Hermes

Rather than interpreting nine charts at once, begin with P → W → E, then P → O, before examining the other methods. Each stage gives you a more specific distinction to investigate.

Use this synthetic sample: 20 March 2000, 12:00:00 UTC, 41°00′ north, 29°00′ east; Gregorian calendar. If a form requires local time, verify the time-zone conversion for that date instead of entering UTC directly as local time.

1. Fix the shared settings. Use Tropical, True Node, Normal cusp setting, Normal zodiac transformation and Normal coordinate view. Record the application version.
2. Record the Placidus starting chart. Note the ASC, MC, all twelve cusps, and the signs and degrees of the planets you will examine. Include any warning.
3. Change only the house method. Examine W, E, R, O, B, T, K and C in turn using the same sample. Recheck the other controls each time.
4. Read the boundary before the planet. If a planet changes house, write down the relevant cusp in each chart. Check separately that the planet’s longitude has remained the same.
5. Return to the starting method. Select Placidus again and compare with your first record. Investigate the input data and other preferences if the values differ.

Keep all twelve cusps in a separate record; this short table is a comparison summary. See the Hermes Chart Settings guide for the controls and the different purposes of saved preferences.

At high latitudes, was your selected method actually used?

At high latitudes, some ecliptic points do not rise or set. This matters to methods built from day and night arcs. The limitations affecting Placidus and Koch around and beyond the polar circles involve computability, as well as the appearance of unusually wide houses.

The Swiss Ephemeris programming manual documents a return of Porphyry cusps together with an error code under the relevant polar-region conditions. A library’s behaviour does not show how Hermes presents a warning. A high-latitude acceptance test in Hermes has not been performed for this article.

Complete the 41° north exercise first. Then make a separate synthetic study using the same UTC and longitude, but change the latitude to 70° north. Examine P and K again. This second study investigates latitude; it is a separate experiment from your original house-method comparison.

Record three things together: the selected method, the method used in the output, and any warning. “Placidus” remaining selected in a menu is not sufficient evidence that the Placidus calculation succeeded. If you cannot establish which method produced the result, mark it as uncertain in your comparison.

What should you record when a planet is near a cusp?

Keep the planet’s exact position and the relevant cusp from both systems when describing a house change. Rounded values on screen may not provide enough precision when the positions are very close. When comparing different programs, also check whether house placement is determined solely from ecliptic longitude or from another positional calculation.

Record a tradition’s interpretive rule—such as treating a planet approaching the next cusp as belonging to that house—separately from the calculated house number. This lets you distinguish a change in boundaries from a change in interpretation.

How should you choose a method for your study?

If you are learning from a particular source, first reproduce its examples with the method it uses. When you introduce another system, choose one question: “Which boundary changed this planet’s house?” or “How did the relationship between the MC and the tenth cusp change?”

Comparing calculations makes the methods visible; it does not, by itself, test the validity of their interpretations. Instead of ranking a system as universally best from a few familiar examples, record the source and purpose behind your choice.

For a first exercise, follow our Hermes walkthrough comparing Placidus and Whole Sign with the same data. One planet, two recorded boundaries and a reason for your choice give you a useful comparison to return to.